The present invention relates to a particulate removing catalyst filter and a particulate removing method. More particularly, the present invention relates to a particulate removing catalyst filter for removing particulates contained in the exhaust gas from a combustion engine, such as a diesel engine or the like, and to a particulate removing method using this catalyst filter.
The exhaust gas discharged from a diesel engine contains particulates. Such particulates contain not only soot (carbon), but also a variety of hydrocarbons from soft hydrocarbons to heavy hydrocarbons (polycyclic aromatic hydrocarbons), sulphuric acid mist (SO.sub.4), and the like. There are some particulates having particle sizes as large as several thousand angstroms. However, it is generally considered that their particle sizes are in a range from about 100 to 1000 angstroms, with the average size being in a range from about 300 to about 500 angstroms.
Such particulates are produced due to incomplete combustion of diesel engine fuel or light oil. Recently, it has been strongly desired to prevent such particulates, as well as nitrogen oxide, from spreading in the atmosphere with a view toward prevention of air pollution.
In this connection, there have been proposed a variety of particulate removing catalyst devices arranged such that a filter carrying an oxidizing catalyst is disposed in the exhaust system of a diesel engine so that catalytic oxidation of the particulates occurs, causing the particulates to be finally decomposed and removed (see for example, Japanese Publication of Unexamined Patent Application No. 185425/1988).
As a typical example of the catalyst filter of the type above-mentioned, there is known a catalyst filter formed in a honeycomb structure, in which the honeycomb structure substrate, made of ceramic or the like, carries a catalyst component, or in which the honeycomb structure itself is made of a catalyst component.
However, the exhaust gas discharged from a diesel engine contains not only the particulates above-mentioned, but also gaseous components such as nitrogen oxide, gaseous hydrocarbons, CO, sulfur dioxide (SO.sub.2) and the like. Accordingly, when the particulates are oxidized and dissolved in or on the partition wall of the honeycomb structure, the sulfur dioxide in the gaseous components is also oxidized and converted into sulfates by the same catalyst. Such sulfates may bring about acid rain or generate secondary particulates. With a view toward prevention of air pollution, it has been strongly desired to minimize the production of such sulfates.
On the other hand, while the engine is rotating at a low speed or with no load applied thereto, the exhaust gas temperature is lower (about 300.degree. C.) than that at loaded normal operation. This prevents the particulates from being sufficiently oxidized and dissolved by the catalyst filter. Accordingly, the particulates are accumulated in the partition walls and thereby prevent the exhaust gas from passing therethrough. This disadvantageously increases the pressure drop of the exhaust gas across the partition walls. If the pressure drop of the exhaust gas across the catalyst filter is increased, the fuel combustion condition in the diesel engine combustion chamber is deteriorated by the back pressure from the catalyst filter. This further accelerates the generation of the particulates, thereby to further increase the accumulation of the particulates on the partition walls. This cycle is exascerbated by the further increase in pressure drop. This involves the likelihood that the fuel combustion will be incomplete in the engine.
To solve such a problem, the catalyst filter may be externally heated so that the oxidation and decomposition of the particulates proceeds sufficiently even though the diesel engine is rotating at a low speed or with no load applied thereto. However, such arrangement requires an external heater and a heat sources therefor. This makes such arrangement unpractical in view of the cost.